It's often the case that ETFs with strong returns see an influx of new investment. For as much as we preach that "past performance doesn't guarantee future returns", a lot of investors chase performance. The ETF industry has grown in leaps and bounds, but not all have enjoyed the ride. That's true even for some ETFs with the track record to back it up.Here are three ETFs that have posted strong returns but that investors have largely still ignored.The Obesity ETF (SLIM)The ETF invests in companies that are involved in the fight against the obesity epidemic. Such companies could include biotechnology, pharmaceutical, health care and medical device companies focused on obesity and obesity-related disease such as diabetes, high blood pressure, cholesterol, heart disease, stroke and sleep apnea.Here's an ETF that's done about everything that could be asked of it. It's beaten the S&P 500. Heck, it's even handily beaten the Nasdaq since the start of 2017.However, total assets in SLIM are still just $11 million. Why the lack of interest? This fund isn't necessarily gimmicky. The obesity epidemic is real and companies are spending millions of dollars to fight (translation: profit from) it. Perhaps obesity isn't as sexy a term as artificial intelligence, robotics or blockchain. I can't say I understand the lack of interest in this one.Global X Millennials Thematic ETF (MILN)MILN is very slowly starting to gain assets going from next to nothing a year ago to about $32 million now, but it's still struggling to remain viable. I figured the "millennial" name alone coupled with the preference for ETFs from millennials themselves would get it some attention, but its adoption has been slow.MILN invests in all the companies you'd expect would be of interest to and used by millennials - Apple (AAPL), Amazon (AMZN), Lowe's (LOW), Netflix (NFLX), Nike (NKE) and Costco (COST). Perhaps this fund has an extraordinarily long tail and will eventually pick up steam. The Robotics Global Robotics & Automation ETF (ROBO) languished in obscurity for three years before taking off in 2017, but it had virtually nothing in the way of returns during that time. MILN does.Buzz U.S. Sentiment Leaders ETF (BUZ)BUZ is an interesting product. It uses AI to identify the most talked about stocks on the internet. It's essentially a trend-focused momentum ETF. Momentum has been a profitable factor to invest in over the past couple of years and this ETF is no exception.BUZ has just $11 million in assets. The CrowdInvest Wisdom ETF (WIZE) attempted a similar strategy and failed miserably. It shut down after just five months having not traded a single share on 75% of the trading days since launch. I think this fund is, pardon the pun, "buzzy" enough to stick around, but investors largely flock to the iShares Edge MSCI USA Momentum Factor ETF (MTUM) if they want to invest in this trend.What do you think? Are these small asset ETFs doomed to fail? Comment down below!